Magnetic shield for integrated circuit packaging

ABSTRACT

Structures and methods for providing magnetic shielding for integrated circuits are disclosed. The shielding comprises a foil or sheet of magnetically permeable material applied to an outer surface of a molded (e.g., epoxy) integrated circuit package. The foil can be held in place by adhesive or by mechanical means. The thickness of the shielding can be tailored to a customer&#39;s specific needs, and can be applied after all high temperature processing, such that a degaussed shield can be provided despite use of strong magnetic fields during high temperature processing, which fields are employed to maintain pinned magnetic layers within the integrated circuit.

RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 10/719,419, filed Nov. 21, 2003, which is a divisional applicationof U.S. application Ser. No. 10/050,339, entitled “MAGNETIC SHIELD FORINTEGRATED CIRCUIT PACKAGING,” filed Jan. 15, 2002, now U.S. Pat. No.6,906,396, the disclosures of which are hereby incorporated by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates to magnetic shielding for integratedcircuits and, more particularly, to magnetic shielding for integratedcircuits having magnetic materials used therein for which protectionfrom stray external magnetic fields is desired.

BACKGROUND OF THE INVENTION

Magnetic materials are used, for example, in magnetic cell memories andmagnetic field sensors. In random access magnetoresistive memories, datais stored by applying magnetic fields and thereby causing a magneticmaterial in a cell to be magnetized into either of two possible memorystates. The information stored in the memory is contained in theorientations of the magnetization vectors of the magnetic materiallayers used in each memory cell. Such memory cells exhibit a pronounceddecrease in electrical resistance when an applied magnetic field bringsthe magnetization vectors in different layers into alignment. Recallingdata is accomplished by sensing resistance changes in the cell. Thecells can be written or erased by applying magnetic fields created bypassing currents through conducting lines external to the magneticstructures, or through the magnetic structures themselves.

There are often undesirable magnetic fields in and about the device,which are generated either as part of the device operation or fromexternal sources. Such fields can have significant effects on themagnetization of the magnetic thin film. The field can contribute to aloss of information or to storage of erroneous information in themagnetic memory cells. Thus, magnetic memory cells function best whenthey are protected from external magnetic field disturbances.

A metal with a relatively high magnetic permeability can be used to forma shield for protection from magnetic fields. Metals that are usedwidely in magnetic shielding include soft magnetic or high permeabilitymaterials, such as NiFe, NiFeMo and NiFeCu. Such magnetic shieldingmaterials, are generally available from metal supply companies, such asCarpenter Technology Corporation of Wyomissing, Pa.

U.S. Pat. No. 5,939,772 entitled “Shielded Package For MagneticDevices,” issued Aug. 17, 1999, describes the use of magneticallypermeable metal shields attached to the outside of a hermetically sealedceramic package. The shields are electrically connected to the packageground plane. Laminated magnetic shielding for ceramic packages is alsodescribed in U.S. Pat. No. 5,561,265, issued Oct. 1, 1996.

Ceramic package technology can be expensive. Furthermore, as performanceincreases, the physical characteristics of ceramic packages may becomelimiting. Specifically, a ceramic material based on Al₂O₃ has arelatively high dielectric constant (ε_(r)˜7-8). Additionally, becauseof the high-temperature processing, metallization is limited torefractory metals that are quite resistive, such as Mo and W.

Other references include application of magnetic shielding within aplastic package. U.S. Pat. No. 4,953,002, issued Aug. 28, 1990, forexample, teaches magnetic shielding internal to a plastic encapsulatedpackage.

Magnetic integrated circuit structures must also be housed in a way thatminimizes cost if they are to be viable for the commercial memorymarket. Therefore, a shielding arrangement to protect magnetic films inmagnetic integrated circuit structures from significant external adverseinfluences, including external magnetic fields, and which can beprovided economically, would be desirable. Desirably, such a shieldingarrangement should be flexible enough to meet the varied needs ofintegrated circuit users.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a housing is providedfor protecting an integrated circuit device. The housing comprises amolded body that encapsulates the integrated circuit device. At leastone magnetically permeable foil is applied to an outer surface of themolded body.

In accordance with another aspect of the invention, a method is providedfor magnetically shielding a semiconductor die. The method includesforming a molded unitary housing around the semiconductor die. A film ofmagnetic shield material is applied to at least one outer surface of themolded unitary housing. The film is applied in a manner that such thatit is approximately parallel to a major surface of the semiconductordie. Advantageously, the shield material can be degaussed just prior toapplication, after the package is subjected to high temperatureprocessing.

In accordance with another aspect of the invention, an integratedcircuit package is provided. The package includes an integrated circuitdie, a molded body encapsulating the die, and a magnetic shield layerextending parallel to a major surface of the die over an outer surfaceof the molded body.

In accordance with still another aspect of the present invention, amethod is provided for packaging an integrated circuit chip. The methodincludes mounting the chip on a die carrier. Epoxy is molded over thechip to form an encapsulant. A magnetic shield layer is then selectedfor a particular integrated circuit environment. This selected magneticshield is applied over the encapsulant.

In accordance with still another aspect of the invention, an integratedcircuit package is provided with an encapsulant surrounding anintegrated circuit die. The encapsulant includes a recess on an outersurface thereof. The recess is configured for receiving and mechanicallyretaining a magnetic shield foil. In the illustrated embodiment, therecess includes overhanging tabs for removably trapping the foil withinthe recess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a packaged integrated circuitwith magnetic shielding attached to outer surfaces of the package,according to an illustrated embodiment of the invention.

FIG. 2 is a schematic cross section of an integrated circuitencapsulated in a ball-grid array package that has magnetic shieldingattached to an outer surface of the package, according to an illustratedembodiment of the invention.

FIG. 3 is a schematic cross section of a packaged integrated circuitwith magnetic shielding set into recesses on outer surfaces of thepackage, according to an illustrated embodiment of the invention.

FIG. 4 is a perspective view of a ball-grid array package showing arecess in the top surface in which a magnetic shield is heldmechanically, according to an illustrated embodiment of the invention.

FIGS. 5A and 5B are schematic cross sections cut along lines 5A-5A and5B-5B, respectively, of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Magnetic integrated circuits, such as MRAM (magnetic random accessmemory) devices, can be sensitive to external magnetic fields.Information is stored in MRAMs specifically as a direction ofmagnetization in a magnetic material layer. If the layer is exposed toan undesirable external magnetic field, the direction of magnetizationcan inadvertently change. Such exposure to stray fields can lead tomemory erasure, accidental writing and/or reading errors.

Of course, the external environments for magnetic integrated circuitdevices are not all the same. Some devices may be located inenvironments with strong external magnetic fields, and some may belocated in environments where external magnetic fields are negligible.When magnetic shielding is incorporated inside the packaging of amagnetic device, a best guess is made as to the size and thickness ofmagnetic shielding to use. There are drawbacks to this “one size fitsall” approach. The designer may choose to provide magnetic shielding fora worst-case scenario, thereby using more magnetic material than may berequired for many applications. In this case, customers pay for moreshielding than they might need. Additionally, customers may wish to haveshielding for only some of their applications.

Perhaps more importantly, magnetic shielding should be degaussed, i.e.,provided with random magnetic orientation. In order to keep the shielddegaussed, the shield should be applied as late as possible in thepackaging process. This is because during any high temperature steps,the chip must be exposed to a controlled magnetic field to ensure thatthe “pinned” or fixed magnetic layers within the chip maintain theirdesired magnetic alignment. Even soldering a package to a circuit boardcan raise temperatures high enough to risk alteration of the pinnedlayers' magnetization. Thus, even packaging steps should be performedunder a controlled magnetic field, if possible. Unfortunately, such afield would also tend to align the magnetic shield, if present, suchthat it would not remain degaussed.

It would be useful to have a system of magnetic shielding for magneticintegrated circuits that can be adapted easily for individual customerapplications, is removable for certain applications and/or can bereadily applied after all high temperature processing, particularlythose steps in which magnetic fields are applied to maintain pinnedlayers within the chip.

The aforementioned needs are satisfied by the embodiments of the presentinvention, which provide package structures and methods for providingmagnetic shielding to an integrated circuit after packaging is complete.Thus, the magnetic shielding can be tailored to meet the specific needsof the customer without incurring the expense of over-shielding or therisk of under-shielding. More importantly, the shield can be degaussedand applied after all high temperature packaging steps. Furthermore, incertain embodiments described herein, magnetic shielding is removablyapplied to an outside surface of an integrated circuit package, suchthat it can be removed and degaussed after packaging and even aftermounting the package without degaussing pinned layers in the chip.

These and other objects and advantages of the present invention willbecome more fully apparent from the following description taken inconjunction with the accompanying drawings.

FIG. 1 is a cross-sectional, schematic drawing of a package or housing10 for an integrated circuit device, according to an illustratedembodiment of the invention. The package comprises a magnetic integratedcircuit 12 encapsulated within a plastic or epoxy encapsulant,preferably in the form of a molded body 14. For purposes of the presentdescription a magnetic integrated circuit is defined as an integratedcircuit containing at least one magnetic thin film layer forming a partof an active device. Preferably, the molded body 14 comprises an organicmaterial, more preferably, an elastomer or an epoxy mold compound. Theskilled artisan will appreciate that the molded body 14 encapsulates thedie 12, in contrast to ceramic packages that are hermetically sealedaround a die.

The integrated circuit 12 is encapsulated onto a die carrier orsubstrate 16. Preferably, the die carrier 16 comprises electricallyconducting leads 18. Conducting wires 20 are bonded to bond pads 22 onthe integrated circuit 12 and attached to the electrically conductingleads 18 of the die carrier 16. In an alternative “flip chip”arrangement (not shown), solder bumps on the integrated circuit arebonded to the leads 18, and conducting wires 20 are not used. In theillustrated embodiment, the leads 18 extend into electrodes 24 thatprotrude from the molded body 14 and can make connections to externalcircuitry. The electrodes 24 typically extend below the molded body 14.

As will be appreciated by the skilled artisan, the features andadvantages described herein will have application to numerous molded orencapsulated integrated circuit packages, such as lead frame packages.More recently, however, die carriers comprise plastic substrates. Forsuch packages, the electrical leads 18 and electrodes 24 representconductive traces on or in a plastic substrate extending out of themolded body 14 to form contacts that eventually form connections withlarger circuits (e.g., a motherboard).

In FIG. 1, magnetically permeable foils 26, 28 are attached to both thetop and bottom outer surfaces of the molded body 14. The foils 26, 28are thus electrically insulated from the packaged circuitry and leads.Preferably, the foils comprise soft magnetic or high permeabilitymaterials, such as nickel-iron based alloys, cobalt-iron based alloys,nickel-cobalt based alloys or amorphous ferromagnetics. More preferably,the foils comprise a NiFe-based alloy, such as mu metal or permalloy.Preferably, the foil thickness is between about 1 μm and 1000 μm. Thefoils 26, 28 are held onto the approximately flat surfaces by thinlayers of adhesive 29, preferably, an epoxy-based adhesive. The foils26, 28 are arranged to be and larger than a major surface of themagnetic integrated circuit 12. In an alternative arrangement, there isa magnetically permeable foil 26 on only one outer surface.

FIG. 2 is a schematic cross section of a ball-grid array housing orpackage 30 for an integrated circuit device 12, according to anotherembodiment of the invention. The integrated circuit or die 12 isattached to a rigid substrate 32 with a die attach material 34,preferably epoxy or elastomer. The rigid substrate 32 containsconductive traces 36 that connect to solder balls 38 arranged in anarray on the bottom surface of the rigid substrate 32. The solder balls38 are configured to make electrical connections to external circuitry.Conductive wires 40 provide conductive paths between bond pads 42 on theintegrated circuit 12 and the conductive traces 36 on the rigidsubstrate 32. A molded body 44 encapsulates the integrated circuit 12onto the rigid substrate 32, with the solder balls 38 serving as theelectrodes that are not covered by the molded body 44 and therefore areexposed on the outside of the package 30. Preferably, the molded body 44comprises an organic material, more preferably, an elastomer or an epoxymold compound.

In FIG. 2, a magnetically permeable foil 46 is attached to an outersurface of the molded body 44, held in place by a thin layer of adhesive48, preferably, an epoxy-based adhesive. The molded body 44 electricallyinsulates the foil 46 from the package circuitry. Preferably, the foilscomprise “soft” magnetic or high permeability materials, such asnickel-iron based alloys cobalt-iron based alloys, nickel-cobalt basedalloys or amorphous ferromagnetics. More preferably, the foils compriseNiFe-based alloys such as mu metal or permalloy. Preferably, the foilthickness is between about 1 μm and 1000 μm. The foil 46 is arranged tobe approximately parallel to and larger than a major surface of themagnetic integrated circuit 12.

FIG. 3 is a cross-sectional, schematic drawing of a housing or package50 for an integrated circuit device 12, according to another embodimentof the invention. The package 50 comprises the magnetic integratedcircuit 12 encapsulated within a molded body 52. Preferably, the moldedbody 52 comprises an organic material, more preferably, an elastomer oran epoxy mold compound.

As described above for FIG. 1, the integrated circuit 12 is encapsulatedby the molded body 52 onto a die carrier 16. Preferably, the carrier 16includes electrically conducting leads 18. Conducting wires 20 arebonded to bond pads 22 on the integrated circuit and attached to theelectrically conducting leads of the die carrier 18, 16. In analternative arrangement (not shown), solder bumps on the integratedcircuit are bonded to electrically conducting traces on a plasticsubstrate in a “flip chip” arrangement, and conducting wires 20 are notused. The electrically conducting leads 18 extend to form electrodes 24that protrude from the molded body 52 and can make connections toexternal circuitry. The electrodes 24 themselves can comprise thecontacts of a lead frame, but more preferably comprise conductive traceson or in a plastic substrate.

In FIG. 3, magnetically permeable foils 54, 56 are fitted into recesses58, 60 in the top and bottom outer surfaces of the molded body 52.Preferably, the foils comprise “soft” magnetic or highly permeablematerials as described hereinabove. The foils 54, 56 are held in placeby thin layers of adhesive 62, preferably, an epoxy-based adhesive. Thefoils 54, 56 are arranged to be approximately parallel to and largerthan a major surface of the magnetic integrated circuit 12. In anotherarrangement, there is a magnetically permeable foil 54 and recess 58 ononly one outer surface of the molded body 52.

In accordance with one arrangement, the recesses 58, 60 are etched intothe encapsulant 52 after molding. Preferably, however, the recesses 58,60 are formed in the body 52 as molded.

Another preferred embodiment for attaching a magnetically permeable foilin a recess in the outer surface of a molded body can be understood withreference to FIG. 4. A finished ball-grid array type of package 70 readyfor the addition of magnetic shielding is shown in a perspective view inFIG. 4. Only the molded body or encapsulant 71 is shown in FIG. 4.

The top surface 72 contains a recessed region 74 over most of its area.The recess 74 has two parallel edges 76 whose sidewalls 78 areapproximately perpendicular to the top surface 72, as is apparent in thecross-sectional view of FIG. 5A. The remaining two parallel edges 80 ofthe recess 74 include an overhanging tab 82 at the top surface 72, whichprotrudes into the region of the recess 74, as is apparent from thecross-sectional view of FIG. 5B. The recess 74 is preferably formed,including overhanging tabs 82, during the molding process. One or moretabs 82 are preferred over a single overhanging ledge extending thelength of the edge 80, simply to facilitate removal of the mold.

FIGS. 5A and 5B show only the top outer surface portion of a housing foran integrated circuit. It will be understood that the outer surfacearrangement shown in FIGS. 5A and 5B can be used with any number ofintegrated circuit and wiring arrangements consistent with molded bodypackages, including those discussed above for FIGS. 1 and 2.Additionally, the outer surface arrangement shown in FIGS. 5A and 5B canbe used either on only one package surface or on both major packagesurfaces, according to the requirements of the operating environment.Preferably, the molded body 71 comprises an organic material, morepreferably, an elastomer or an epoxy mold compound.

FIG. 5A is a cross section of the recess 74 cut through the recess edges76 whose sidewalls 78 are approximately perpendicular to the top surface72 of the molded body 71. A sheet of magnetic shield material 84 lieswithin the recess 74 with its edges 86 adjacent to the sidewalls 78 ofthe recess 74.

FIG. 5B is a cross section of the recess 74 cut along a surfaceperpendicular to the surface shown in FIG. 5A. The top edges 80 of therecess 74 have at least one overhanging tab 82 at the top surface 72 ofthe housing 70 and, deeper inside the recess 74, sidewalls 88 that areapproximately perpendicular to the plane of the top surface 72. Theoverhanging tabs 82 protrude into the region of the recess 74. A sheetof magnetic shield material 84 is trapped within the recess 74, belowthe tabs 82, with its edges 90 adjacent to the sidewalls 88 of therecess 74. It will be understood that, in other arrangements, the tabs82 can taper to the recess floor rather than having the illustratedperpendicular sections 88. The illustrated tab configuration, taperingabove and below the innermost protrusion, facilitates deflection toinsert and/or remove the magnetic shield 84.

In the illustrated embodiment, no adhesive is used to hold the sheet ofmagnetic shield material 84 in place within the recess 74 of the moldedbody 71 for the magnetic integrated circuit. The sheet of magneticshield material 84 is cut to fit the size of the recess 74. The sheet 84is placed into the recess 74 by bending the sheet 84 slightly to fitunder the overhangs 82 and then releasing the sheet 84 to fit into placeagainst the sidewalls 88 of the recess 74. The width of the recessopening within the overhang edges 82 is less than the width of themagnetic material sheet 84, thus providing a mechanical means of keepingthe magnetic material sheet 84 in place. It will be understood that, ifdesired, adhesive can additionally be employed.

Advantageously, the magnetic shield 84 can additionally be removed andreplaced. Thus, a package can be shipped with the shield 84 in place.The customer can remove the shield 84, conduct additional hightemperature processing in a strong magnetic field (without affecting theshield), and replace the shield after completion of high temperaturepackaging steps. Alternatively, after installation and use, the shield84 can be removed for degaussing again, should the need arise.

The embodiments of the invention have been described using examples ofpackages that contain one integrated circuit or die. The embodiments ofthe invention are equally useful for a multi-die package, whereinintegrated circuits are arranged next to one another and/or stacked oneover another within one molded package. Connections among the dies andbetween the dies and conducting traces connected to electrodes thatprotrude from the package can be made by wire bonding or by solder bumpbonding as described above with respect to the illustrated embodiments.

The structures and methods described above in the illustratedembodiments offer many advantages for magnetic shielding of magneticintegrated circuits. Fully processed and packaged integrated circuitdevices can be removed from the fab environment and inventoried. At thispoint, all high temperature processing has been completed. Magneticshielding, tailored to meet a particular customer's requirements, can beadded to the outside of the packages just prior to shipping. Themagnetic shielding is preferably degaussed and/or given a particularmagnetic alignment according to customer needs. This would not bepossible if the magnetic shielding were introduced into the integratedcircuit or the package before all high temperature processing wascomplete. Moreover, the embodiments described herein obtain magneticshielding, post-processing tailoring and the benefits of low-dielectricepoxies and high conductivity copper metallization for IC packaging.

Although the foregoing description of the preferred embodiments of thepresent invention has shown, described and pointed out the fundamentalnovel features of the invention, it will be understood that variousomissions, substitutions and changes in the form of the detail of theapparatus as illustrated as well as the uses thereof may be made bythose skilled in the art, without departing from the spirit of thepresent invention. Consequently, the scope of the present inventionshould not be limited to the foregoing discussion, but should be definedby the appended claims.

1. A method of magnetically shielding a semiconductor die, comprising:forming a molded unitary housing around the semiconductor die; andapplying a preformed film of magnetic shield material to at least oneouter surface of the molded unitary housing, the preformed film beingapproximately parallel to a major surface of the semiconductor die. 2.The method of claim 1, wherein forming a molded unitary housingcomprises encapsulating a plurality of semiconductor dies.
 3. The methodof claim 1, wherein the at least one outer surface of the molded unitaryhousing comprises a recessed region, into which region the preformedfilm of magnetic shield material is applied.
 4. The method of claim 1,wherein applying the preformed film of magnetic shield material to atleast one outer surface of the molded unitary housing comprises applyingthe preformed film to both a top outer surface and a bottom outersurface of the molded unitary housing.
 5. The method of claim 1, whereinthe semiconductor die is attached to a plastic substrate before themolded unitary housing is formed, and the molded unitary housingencapsulates the semiconductor die on the plastic substrate.
 6. Themethod of claim 5, wherein the plastic substrate comprises a ball gridarray substrate.
 7. The method of claim 5, further comprising bondingwires between the semiconductor die and electrical traces on the plasticsubstrate after the semiconductor die is attached to the plasticsubstrate and before forming the molded unitary housing.
 8. The methodof claim 5, further comprising bonding solder bumps on the semiconductordie to electrical traces on the plastic substrate before forming themolded unitary housing.
 9. The method of claim 1, wherein applying thepreformed film of magnetic shield material to at least one outer surfaceof the molded unitary housing comprises attaching the preformed film tothe molded unitary housing with an epoxy-based adhesive.
 10. The methodof claim 1, wherein the magnetic shield material is selected from thegroup consisting of mu metal and permalloy.
 11. The method of claim 1,wherein applying the preformed film of magnetic shield material isconducted after all high temperature processing.
 12. The method of claim1, further comprising degaussing the preformed film of magnetic shieldmaterial before applying the preformed film to the at least one outersurface of the molded unitary housing.
 13. The method of claim 12,further comprising removing the preformed film of magnetic material fromthe outer surface of the molded unitary housing before degaussing andre-applying the preformed film.
 14. The method of claim 1, whereinapplying the preformed film of magnetic shield material furthercomprises retaining the preformed film of magnetic shield materialwithin a recess formed in the molded unitary housing.
 15. A method ofpackaging an integrated circuit chip, comprising: mounting the chip on adie carrier; molding epoxy over the chip to form an encapsulant;selecting a preformed magnetic shield layer having a thickness tailoredto a strength of an external magnetic field of an intended environment;and applying the selected preformed magnetic shield layer over theencapsulant.
 16. The method of claim 15, further comprising forming arecess in a major surface of the encapsulant, wherein applying comprisesfitting the selected preformed magnetic shield layer within the recess.17. The method of claim 16, further comprising removing the selectedpreformed magnetic shield layer from the recess, conducting hightemperature processing upon the packaged chip while the preformedmagnetic shield layer is removed, and replacing the magnetic shieldlayer after high temperature processing.
 18. The method of claim 17,further comprising applying a strong magnetic field to the packaged chipduring the high temperature processing.
 19. The method of claim 15,wherein applying comprises adhering.